Catheter with strain gauge sensor

ABSTRACT

A medical probe, including a flexible insertion tube, having a distal end for insertion into a body cavity of a patient and which is configured to be brought into contact with tissue in the body cavity. The probe further includes a sensor tube of an elastic material, contained inside the distal end of the insertion tube and configured to deform in response to forces exerted by the tissue on the distal end. The probe also includes a plurality of strain gauges fixedly attached to a surface of the sensor tube at different, respective locations and configured to generate respective signals in response to deformations of the sensor tube.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority to,co-pending U.S. application Ser. No. 12/647,824, filed on Dec. 28, 2009,which is incorporated in its entirety herein by explicit reference.

FIELD OF THE INVENTION

The present invention relates generally to invasive medical devices, andspecifically to the construction of probes for insertion into bodyorgans.

BACKGROUND OF THE INVENTION

In some diagnostic and therapeutic techniques, a catheter is insertedinto a chamber of the heart and is brought into contact with the innerheart wall. In such procedures, it is generally important that thedistal tip of the catheter engages the endocardium with sufficientpressure to ensure good contact. Excessive pressure, however, may causeundesired damage to the heart tissue and even perforation of the heartwall.

For example, in intracardiac radio-frequency (RF) ablation, a catheterhaving an electrode at its distal tip is inserted through the patient'svascular system into a chamber of the heart. The electrode is broughtinto contact with a site (or sites) on the endocardium, and RF energy isapplied through the catheter to the electrode in order to ablate theheart tissue at the site. Proper contact between the electrode and theendocardium during ablation is necessary in order to achieve the desiredtherapeutic effect without excessive damage to the tissue.

A number of patent publications describe catheters with integratedpressure sensors for sensing tissue contact. As one example, U.S. PatentApplication Publication 2007/0100332 to Saurav et al., whose disclosureis incorporated herein by reference, describes systems and methods forassessing electrode-tissue contact for tissue ablation. Anelectro-mechanical sensor within the catheter shaft generates electricalsignals corresponding to the amount of movement of the electrode withina distal portion of the catheter shaft. An output device receives theelectrical signals for assessing a level of contact between theelectrode and a tissue.

The description above is presented as a general overview of related artin this field and should not be construed as an admission that any ofthe information it contains constitutes prior art against the presentpatent application.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a medical probe,including:

a flexible insertion tube, having a distal end for insertion into a bodycavity of a patient and which is configured to be brought into contactwith tissue in the body cavity;

a sensor tube including an elastic material, contained inside the distalend of the insertion tube and configured to deform in response to forcesexerted by the tissue on the distal end; and

a plurality of strain gauges fixedly attached to a surface of the sensortube at different, respective locations and configured to generaterespective signals in response to deformations of the sensor tube.

Typically, the sensor tube is fixedly attached inside the distal end tothe insertion tube, and the surface of the sensor tube may include anouter curved surface of the sensor tube.

In one embodiment the respective signals are configured to generate amagnitude and a direction of a pressure on a termination of the distalend. Alternatively or additionally, the respective signals areconfigured to generate a magnitude and a direction of a deflection of atermination of the distal end.

The plurality of strain gauges may be symmetrically disposed withrespect to an axis of the sensor tube, and additionally may bepositioned on a circumference of the surface of the sensor tube, whereinthe circumference is centrally located with respect to the sensor tube.

In a disclosed embodiment the plurality of strain gauges have respectivegauge directions, and the gauges are fixedly attached to the surface ofthe sensor tube so that the respective gauge directions are parallel toan axis of the sensor tube.

In a further disclosed embodiment the probe includes at least onetemperature-compensating strain gauge configured to generate signals tocompensate for changes in temperature of the plurality of strain gauges.The at least one temperature-compensating strain gauge may be mounted ona block having a block-thermal mass, and the sensor tube may have anassembly-tube-thermal mass equal to the block-thermal mass.

The at least one temperature-compensating strain gauge may be mounted onthe block in a location that does not deform in response to deformationsof the sensor tube.

The at least one temperature-compensating strain gauge may have atemperature-compensating strain gauge direction configured to beorthogonal to an axis of the sensor tube.

There is further provided, according to a disclosed embodiment of thepresent invention, a method for producing a medical probe, including:

providing a flexible insertion tube, having a distal end for insertioninto a body cavity of a patient and which is configured to be broughtinto contact with tissue in the body cavity;

installing a sensor tube including an elastic material inside the distalend of the insertion tube, the sensor tube being configured to deform inresponse to forces exerted by the tissue on the distal end; and

fixedly attaching a plurality of strain gauges to a surface of thesensor tube at different, respective locations, the strain gauges beingconfigured to generate respective signals in response to deformations ofthe sensor tube.

There is further provided, according to a disclosed embodiment of thepresent invention, apparatus for performing a medical procedure,including:

a medical probe, including:

a flexible insertion tube, having a distal end for insertion into a bodycavity of a patient and which is configured to be brought into contactwith tissue in the body cavity;

a sensor tube including an elastic material, contained inside the distalend of the insertion tube and configured to deform in response to forcesexerted by the tissue on the distal end; and

a plurality of strain gauges fixedly attached to a surface of the sensortube at different, respective locations and configured to generaterespective signals in response to deformations of the sensor tube; and

a console, which receives the respective signals from the strain gaugesand in response provides an indication of the forces on the distal end.

There is further provided, according to a disclosed embodiment of thepresent invention, a method for performing a medical procedure,including:

providing a flexible insertion tube, having a distal end for insertioninto a body cavity of a patient and which is configured to be broughtinto contact with tissue in the body cavity;

installing a sensor tube including an elastic material inside the distalend of the insertion tube, the sensor tube being configured to deform inresponse to forces exerted by the tissue on the distal end;

fixedly attaching a plurality of strain gauges to a surface of thesensor tube at different, respective locations, the strain gauges beingconfigured to generate respective signals in response to deformations ofthe sensor tube;

receiving the respective signals at a console; and

providing at the console an indication of the forces on the distal endin response to receiving the respective signals.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of operation of a catheter, accordingto an embodiment of the present invention;

FIG. 2A is a schematic, cutaway view of the catheter, showing a pressuresensing assembly, and FIG. 2B is a schematic perspective view of thepressure sensing assembly, according to an embodiment of the presentinvention;

FIG. 3A is a schematic, cutaway view of the catheter, showing analternative pressure sensing assembly, and FIG. 3B is a schematicperspective view of the alternative pressure sensing assembly, accordingto an embodiment of the present invention;

FIG. 4 is a schematic perspective view of a further alternative pressuresensing assembly, according to an embodiment of the present invention;and

FIG. 5 is a schematic perspective view of a yet further alternativepressure sensing assembly, according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OVERVIEW

Embodiments of the present invention provide a novel design of aninvasive probe, such as a catheter. The probe comprises a flexibleinsertion tube for insertion into a body cavity of a patient, and apressure sensing assembly is installed inside the distal end of theinsertion tube. The assembly comprises a tube of elastic material havinga plurality of strain gauges fixedly attached to different respectivelocations of a surface, typically the outer curved surface, of the tube.Typically, three or more strain gauges are fixedly attached to thesurface.

When the distal end of the probe engages tissue in the body cavity,force on the end causes the probe to bend and/or compress, and the tubeof elastic material to deform. Depending on the magnitude and directionof the force relative to the tube, the deformation of the tube consistsof expansion and/or contraction of the locations of the tube's surfaceto which the strain gauges are attached. The strain gauges providerespective signals in response to the deformation of the locations ofthe tube to which the gauges are attached, and a processing unitcalculates the pressure (caused by the force) on the end, from thestrain gauge signals. The processing unit may also calculate adeflection of the end from the signals.

The combination of strain gauges attached to an elastic tube provides areliable, accurate, pressure sensing device that is simpler and cheaperto manufacture than other pressure sensing devices known in the art.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of operation of a catheter 20,according to an embodiment of the present invention. As is illustratedin FIG. 1, a proximal end of catheter 20 is coupled to an operatingconsole 21, which is controlled by an operator of the catheter. Theoperator also manipulates the catheter using controls (not shown) whilethe catheter is being used to perform a medical procedure. Console 21comprises a processing unit (PU) 23, which, inter alia, receives andanalyzes signals from elements at a distal end of catheter 20. Theelements and the signal analysis are described in more detail below.Processing unit 23 uses software stored in a memory 25, the softwaretypically including a look-up table 27, for the signal analysis, as wellas for performing other functions related to operation of the catheter.The software may be downloaded to console 21 in electronic form, over anetwork, for example, or it may, alternatively or additionally, beprovided and/or stored on tangible media, such as magnetic, optical, orelectronic memory.

Catheter 20 comprises a flexible insertion tube 26. A lower portion ofFIG. 1 schematically illustrates a sectional view of a chamber of aheart 22, showing flexible insertion tube 26 of the catheter 20 insidethe heart. The catheter is typically inserted into the heartpercutaneously through a blood vessel, such as the vena cava or theaorta. A termination 28 of tube 26, fixedly attached to a distal end 24of the tube, engages endocardial tissue 30. Typically termination 28comprises an electrode, which may be used for ablation. However, thereis no requirement that termination 28 comprises an electrode and thetermination may comprise any other element, such as a camera. Forsimplicity, in the following description termination 28 is assumed tocomprise an electrode, and those having ordinary skill in the art willbe able to adapt the description, mutatis mutandis, for other types oftermination. Force exerted by termination 28 against the endocardiumdeforms the endocardial tissue locally, and leads to a countervailingforce from the endocardium on the termination.

In the pictured example, termination 28 engages the endocardium head-on.The countervailing force causes distal end 24 of insertion tube 26 tocompress slightly. Alternatively (not pictured), the termination engagesthe endocardium at an angle. In this case, the countervailing force fromthe endocardium bends distal end 24 of the tube, and may also compressthe distal end. As is explained in more detail below, embodiments of thepresent invention measure either type of deformation, due to head-on orangled engagement, and the measurements provide an indication of thedirection and magnitude of the force, and also of the pressure, causingthe deformation. (The measurements may also provide an indication of thedeflection of termination 28.)

The pressure indication may be used by the operator of catheter 20 toensure that termination 28 is pressing against the endocardium firmlyenough, and in a required direction, to give the desired result, but notso hard or in a direction as to cause undesired tissue damage. U.S.Patent Application 20090093806, to Govari et al., filed Oct. 8, 2007,whose disclosure is incorporated herein by reference, describes a systemthat uses a pressure-sensing catheter in a similar manner. Catheter 20may be used in such a system.

The deflection indication may be used by the operator to judge iftermination 28 is correctly positioned.

FIG. 2A is a schematic, cutaway view of catheter 20, showing a pressuresensing assembly 40 installed in distal end 24 of insertion tube 26, andFIG. 2B is a schematic perspective view of the pressure sensingassembly, according to an embodiment of the present invention. Pressuresensing assembly 40 comprises a thin-walled tube 42 of an elasticmaterial, typically a metal material, and the tube is herein alsoreferred to as sensor tube 42. The sensor tube is typically in the formof a hollow right circular cylinder, having an axis of symmetry 46. Theelastic material forming the tube may comprise a superelastic alloy suchas nickel titanium (Nitinol). For intracardiac applications, the overalllength of tube 42 may be approximately 5 mm, with a wall thickness ofapproximately 0.1 mm, and an outer diameter of approximately 2 mm.Alternatively, in other applications, the dimensions of sensor tube 42may be larger or smaller than those exemplified above.

For clarity, in the following description, sensor tube 42 is assumed tobe oriented and aligned with a set of xyz orthogonal axes, as shown inFIG. 2A, (the x- and y-axes being in the plane of the paper) so thataxis 46 is parallel to the x-axis. However, it will be appreciated thatembodiments of the present invention may function in substantially anyorientation.

A plurality of strain gauges 44A, 44B, 44C, . . . are fixedly attachedto a surface 48 of sensor tube 42. In the following description, exceptwhere otherwise indicated, surface 48 is assumed to comprise the outercurved surface of tube 42. Strain gauges 44A, 44B, 44C, . . . are alsoreferred to herein generically as strain gauges 44 and are also referredto as strain-measuring gauges 44.

Typically, strain gauges 44 are of generally similar types. A number ofdifferent types of strain gauges, i.e., gauges that provide anindication of the amount of deformation of a material, are known in theart. For example, strain gauges 44 may comprise metallic foil gauges orsemiconductor gauges (the latter using a piezoresistive effect), whichrespectively indicate strain by a change in resistance of a metal or asemiconductor. Alternatively, strain gauges 44 may comprise otherdevices capable of providing an indication of strain, such as apiezoelectric crystal, which indicates strain according to a potentialdeveloped across the crystal in response to the strain, or a fiberoptic, which indicates strain by a change in a characteristic of thefiber optic, such as a resonant wavelength of a diffraction gratingformed in the fiber optic. The fiber optic characteristic is typicallyevaluated by radiation transmitted through the fiber optic.

In the description and in the claims, the terms strain gauge andstrain-measuring gauge are assumed to comprise any device, such as thoseexemplified above, which is able to provide an indication of strain. Forsimplicity, except where otherwise indicated, in the followingdescription strain-measuring gauges 44 are assumed to comprise metallicfoil gauges.

Strain gauges typically measure strain in a particular direction, hereintermed the “gauge direction,” relative to the gauge. In the figures, thegauge direction for a particular gauge is indicated by a double-headedarrow adjoining the symbol for the gauge. Thus, by way of example and asshown in FIGS. 2A and 2B, strain gauges 44 have a common alignment, sothat their gauge directions are parallel to the x-axis. However, thereis no requirement that the gauge direction of strain gauges 44 have acommon alignment, and in some embodiments the gauge directions aredifferent.

In the following description of a disclosed embodiment, there areassumed to be three strain-measuring gauges 44A, 44B, and 44C, which aresymmetrically disposed with respect to sensor tube 42. It will beunderstood that such a symmetric disposition of the strain gauges withrespect to the sensor tube facilitates determining the magnitude anddirection of the pressure exerted on termination 28, as well as themagnitude and direction of the deflection of the termination, regardlessof the direction of the pressure. Also in the disclosed embodiment,strain gauges 44A, 44B, and 44C are fixedly attached to tube 42 on acommon circumference 50 of an outer surface of the tube. Commoncircumference 50 is typically approximately centrally positioned withrespect to tube 42, and is orthogonal to axis 46 so that it lies in a yzplane. It will be understood that central positioning of circumference50 typically enhances values read from gauges 44 for a given pressure ontermination 28.

The attachment of the gauges may be implemented using any convenientbonding material, such as a cyanoacrylate based adhesive. The gauges aretypically attached so that they are disposed symmetrically oncircumference 50, and so that their gauge directions are as describedabove, i.e., parallel to the x-axis.

However, in other embodiments of the present invention, the plurality ofstrain-measuring gauges 44 may comprise any integral number greater thanor equal to 2, and there is no requirement that the plurality bedisposed symmetrically with respect to tube 42. An embodiment with anasymmetric disposition of strain gauges is described with reference toFIG. 5 below.

Each strain gauge 44 is typically connected by respective cabling toprocessing unit 23 (FIG. 1). In FIG. 2A, cabling 52 to gauge 44B andcabling 54 to gauge 44C are shown. The cabling to strain gauges 44 isalso referred to herein generically as cabling 55. Depending on the typeof strain gauge, cabling 55 may typically comprise a twisted-pair cableor a fiber optic cable. Signals are transferred via the cabling toprocessing unit 23, which is configured to analyze the signals so as todetermine the strain of each gauge.

Insertion tube 26 of the catheter comprises an outer flexible tubularsheath 60, which encloses operative portions of the catheter. Theoperative portions include elements such as conductive cabling and/orfluid-carrying tubing and/or pull-wires to guide catheter 20. Forsimplicity, these operative portions of the catheter are not shown inFIG. 2A. Sheath 60 typically comprises a composition of plastic withstrengthening elements, such as metallic wiring, included in thecomposition.

Pressure sensing assembly 40 is fixedly attached to sheath 60. By way ofexample, the attachment is assumed to be implemented by two generallysimilar rings 56, 58. Rings 56, 58 have internal diameters equal to theexternal diameter of sensor tube 42, and external diameters equal to theinternal diameter of sheath 60. Rings 56, 58 are typically positioned atrespective ends of the sensor tube. During positioning, assembly 40 maybe fixedly attached to sheath 60 by application of an adhesive or cementto the internal and external circumferences of rings 56, 58. Rings 56,58 are typically configured so that, on attachment of assembly 40 tosheath 60, the assembly is symmetrically disposed with respect to thesheath, i.e., with respect to distal end 24.

Rings 56, 58 are one possible method by which assembly 40 is fixedlyattached to sheath 60. Those having ordinary skill in the art willunderstand that other methods for attachment, for example using aplurality of spacers between the assembly and the sheath, may be usedinstead of, or in addition to, rings 56, 58. All such methods areassumed to be comprised within the scope of the present invention.

Once assembly 40 has been fixedly installed in distal end 24, theassembly may be calibrated, prior to the catheter being used for aninvasive procedure such as that described above. The calibrationcomprises applying forces, that are known in both magnitude anddirection, to distal end 24, typically by applying such forces totermination 28. In the following description, it is assumed that theknown forces are converted to corresponding known pressures, using aneffective area of termination 28 upon which the forces are applied. Theapplied known pressures include ranges of head-on pressures, as well asranges (in magnitude and direction) of angled pressures, and thepressures cause corresponding deformations in sensor tube 42. For eachknown pressure, measurements of the strain generated by gauges 44attached to the sensor tube are recorded and stored as calibrationparameters of catheter 20, for example as look-up table 27 in memory 25.

The applied known pressures generate corresponding respectivedeflections of termination 28. The deflections may be measured, in bothdirection and magnitude, and are assumed, by way of example, to also beincorporated in the calibration parameters of catheter 20.

It will be understood that the calibration procedure described above,i.e., generating a look-up table, is one of a number of methods that maybe used to calibrate assembly 40.

Other methods for calibrating the assembly will be familiar to thosehaving ordinary skill in the art. For example, a first function relatingthe magnitude of the pressure to the values of strain of gauges 44, anda second function relating the direction of the pressure to the valuesof strain of the gauges 44, may be formulated. Typically, both functionsare in the form of polynomials having the values of the strains ofgauges 44, as well as higher powers of the strain values, as variablesof the polynomials, each variable being multiplied by a respectivecoefficient. Similar functions may be formulated for the deflections oftermination 28. The calibration procedure described above is used todetermine the values of the coefficients for the respective functions,typically by applying a fitting procedure to the functions.

The equations and coefficient values determined in the calibration maybe stored in memory 25 as the calibration parameters referred to above.

All calibration methods such as those described above are assumed to becomprised within the scope of the present invention.

During an invasive procedure such as is exemplified above, processingunit 23 determines “raw” values of strain measured by each of gauges 44.The processing unit accesses the calibration parameters stored in memory25, in order to evaluate from the raw strain values, values of themagnitude and the direction of the pressure on termination 28, as wellas values of the magnitude and direction of the deflection of thetermination. The determined values may be provided to the operator ofcatheter 20, typically as part of a graphic user interface displayed onconsole 21.

The determination of the values of the magnitude and the direction ofthe pressure, and of the deflection, by processing unit 23 typicallyinvolve operations on the raw values, such as smoothing and/orfiltering. In addition, depending on the type of calibration parameters,further operations may be required, For example, if the calibrationcomprises look-up table 27, processing unit 23 may apply an operationsuch as interpolation or extrapolation to determine, from the measuredstrain values of gauges 44, the direction and magnitude of the pressureon termination 28, and/or of the deflection of the termination. All suchoperations will be familiar to those having ordinary skill in the art,and are assumed to be included within the scope of the presentinvention.

FIG. 3A is a schematic, cutaway view of catheter 20, showing a pressuresensing assembly 140 installed in distal end 24 of insertion tube 26,and FIG. 3B is a schematic perspective view of the pressure sensingassembly, according to an alternative embodiment of the presentinvention. Apart from the differences described below, the operation ofassembly 140 is generally similar to that of assembly 40 (FIGS. 2A and2B), and elements indicated by the same reference numerals in bothassemblies 40 and 140 are generally similar in construction and inoperation.

Readings from strain gauges 44 may be sensitive to temperature changesof the gauges. Because of its dimensions, sensor tube 42 has a lowthermal mass, so that any changes in the ambient temperature around thetube typically cause corresponding changes in the temperature of gauges44, affecting the accuracy of the values read from the gauges. Assembly140 enables processing unit 23 to compensate for these changes, byincorporating one or more temperature-compensating strain gauges 142 inthe assembly. Typically, gauges 142 are the same type of gauges asgauges 44, and are coupled by cabling similar to cabling 55 to PU 23.For simplicity, in FIGS. 3A and 3B only one temperature-compensatinggauge 142 without cabling is shown.

In the alternative embodiment described herein, temperature-compensatinggauges 142 are assumed to be mounted on a block 144, which has a thermalmass that is typically approximately the same as that of sensor tube 42.However, block 144, in contrast to tube 42, is configured and positionedin assembly 140 to be relatively inflexible when distal end 24 deforms.In addition, the gauge direction of temperature-compensating gauges 142are typically set to be orthogonal to the gauge directions of gauges 44,so that in the embodiment described herein, the gauge directions ofgauges 142 are orthogonal to axis 46. Thus, temperature-compensatinggauges 142 have approximately the same temperature changes as gauges 44,but do not respond to the strains measured by gauges 44.

By way of example, block 144 is assumed to be a disk, having an externaldiameter equal to the internal diameter of tube 42. The block may befixedly attached to sensor tube 42, for example, as illustrated in FIGS.3A and 3B, by being positioned at the proximal end of the sensor tube,with a disk axis 148 congruent with axis 46. This arrangementfacilitates the positioning of temperature-compensating gauges 142 sothat their gauge directions are orthogonal to the gauge directions ofgauges 44. Typically, block 144 comprises one or more apertures 146,which may be used to allow passage of elements such as cabling throughsensor tube 42. In addition, the dimensions of apertures 146 may beadjusted so that the thermal mass of block 144 is approximately the sameas that of tube 42.

Assembly 140 is calibrated substantially as described above for assembly40. However, rather than using the “raw” values from strain gauges 44,as are used by processing unit 23 to calibrate assembly 40, theprocessing unit typically uses values of differences betweenstrain-measuring gauges 44 and temperature-compensating strain gauges142 to generate calibration parameters for assembly 140. Once thecalibration of assembly 140 has been performed, during a procedure suchas is described above, processing unit 23 typically uses differencevalues between the two types of gauges, and the assembly 140 calibrationparameters, to determine the magnitude and direction of the pressurebeing applied to termination 28, as well as the deflection of thetermination.

FIG. 4 is a schematic perspective view of a pressure sensing assembly240, according to a further alternative embodiment of the presentinvention. Apart from the differences described below, the operation ofassembly 240 is generally similar to that of assembly 140 (FIGS. 3A and3B), and elements indicated by the same reference numerals in bothassemblies 140 and 240 are generally similar in construction and inoperation. Assembly 240 is typically installed within distal end 24 ofcatheter 20, substantially as is described above for assembly 140 (FIG.3A).

In assembly 240, in contrast to assembly 140, temperature-compensatinggauges 142 are not mounted on block 144, and the block does not formpart of assembly 240. Rather, in assembly 240, gauges 142 are mountedon, or in proximity to, a region of sensor tube 42 that is not subjectto the strains measured by gauges 44. Such regions, by way of example,comprise the ends of sensor tube 42, where the tube is fixedly attachedto sheath 60 by rings 56, 58 (FIGS. 3A and 3B). Thus, as is exemplifiedby one temperature-compensating gauge 142 illustrated in FIG. 4, gauges142 may be mounted on an internal curved surface 49 of sensor tube 42,close to the proximal end of the tube. Alternatively or additionally, atleast some gauges 142 may be mounted on outer curved surface 48, closeto the proximal end of the sensor tube; in some cases the mounting maybe beneath rings 56 or 58 by providing a recess in the rings.

As for assembly 140, in assembly 240 temperature-compensating gauges 142are typically oriented so that their gauge directions are orthogonal tothe gauge directions of strain gauges 44.

FIG. 5 is a schematic perspective view of a pressure sensing assembly340, according to a yet further alternative embodiment of the presentinvention. Apart from the differences described below, the operation ofassembly 340 is generally similar to that of assembly 40 (FIGS. 2A and2B), and elements indicated by the same reference numerals in bothassemblies 40 and 340 are generally similar in construction and inoperation. Assembly 340 is typically installed within distal end 24 ofcatheter 20, substantially as is described above for assembly 40 (FIG.2A).

By way of example, in assembly 340 strain-measuring gauges 44 areassumed to comprise five gauges 44D, 44E, 44F, 44G, and 44H. In contrastto assembly 40, in assembly 340 the five strain gauges 44 are notmounted on sensor tube 42 symmetrically. Rather, gauges 44D, 44E, 44F,44G, and 44H are mounted in an asymmetric manner on the tube. Such anasymmetric disposition of gauges 44 with respect to the sensor tube mayallow greater flexibility in positioning the strain gauges compared to asymmetric arrangement. Typically, gauges 44 are evenly distributed oneither side of circumference 50, i.e., in a positive or a negativedirection parallel to axis 46, with respect to the circumference.However, there is no necessity for such an even distribution, and anuneven distribution may be implemented in some embodiments.

The angles subtended by adjacent strain gauges 44 with axis 46, measuredorthogonally to the axis, may be set to be approximately equal. Theequalization of the angles facilitates determining the pressure exertedon termination 28 regardless of the direction of the pressure, as wellas facilitating determining the deflection of the termination regardlessof the pressure direction.

In some embodiments of the present invention, it may be desirable tomeasure the pressure exerted on termination 28, and/or the termination'sdeflection, if the pressure is in one or more predetermined directions.For example, catheters known in the art have pull-wires that bend thecatheter in one or more particular predetermined directions. Embodimentsof the present invention allow for measuring the pressure and thedeflection in a predetermined direction by appropriate positioning ofgauges 44.

For example, gauges 44E and 44D may be respectively repositioned tolocations 342 and 344 on surface 48 of tube 42, the locations beingselected so that gauges 44E, 44F, and 44D lie on a line 346 of surface48, where line 346 is parallel to axis 46. Such an arrangement of gaugestypically enables processing unit 23 to measure deformation of distalend 24 in predetermined directions, defined by being within the planecomprising line 346 and axis 46, more accurately than if gauges 44E and44D are not repositioned. The pressure on, and/or the deflection of,termination 28 may consequently be measured more accurately. Otherarrangements of gauges 44 on tube 42, for measurement of deformation ofdistal end 24 in a predetermined direction, will be apparent to thosehaving ordinary skill in the art, and all such arrangements are assumedto be within the scope of the present invention.

It will be understood that features described above may be combined inways that are not specifically described. As a first example,temperature-compensating gauges, such as are described with reference toassemblies 140 and 240, may be incorporated into assembly 340. As asecond example, strain-measuring gauges, in assemblies such as assembly340, may be arranged so that they are able to measure pressuresubstantially regardless of direction, and additionally or alternativelyin one or more predetermined directions.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

What is claimed is:
 1. A medical probe, comprising: a flexible insertiontube, having a distal end for insertion into a body cavity of a patientand which is configured to be brought into contact with tissue in thebody cavity; a sensor tube comprising an elastic material, containedinside the distal end of the insertion tube and configured to deform inresponse to forces exerted by the tissue on the distal end; and aplurality of strain gauges fixedly attached to a surface of the sensortube at different, respective locations and configured to generaterespective signals in response to deformations of the sensor tube. 2.The probe according to claim 1, wherein the sensor tube is fixedlyattached inside the distal end to the insertion tube.
 3. The probeaccording to claim 1, wherein the surface of the sensor tube comprisesan outer curved surface of the sensor tube.
 4. The probe according toclaim 1, wherein the respective signals are configured to generate amagnitude and a direction of a pressure on a termination of the distalend.
 5. The probe according to claim 1, wherein the respective signalsare configured to generate a magnitude and a direction of a deflectionof a termination of the distal end.
 6. The probe according to claim 1,wherein the plurality of strain gauges are symmetrically disposed withrespect to an axis of the sensor tube.
 7. The probe according to claim1, wherein the plurality of strain gauges are positioned on acircumference of the surface of the sensor tube, and wherein thecircumference is centrally located with respect to the sensor tube. 8.The probe according to claim 1, wherein the plurality of strain gaugeshave respective gauge directions, and wherein the gauges are fixedlyattached to the surface of the sensor tube so that the respective gaugedirections are parallel to an axis of the sensor tube.
 9. The probeaccording to claim 1, and comprising at least onetemperature-compensating strain gauge configured to generate signals tocompensate for changes in temperature of the plurality of strain gauges.10. The probe according to claim 9, wherein the at least onetemperature-compensating strain gauge is mounted on a block having ablock-thermal mass, and wherein the sensor tube has anassembly-tube-thermal mass equal to the block-thermal mass.
 11. Theprobe according to claim 10, wherein the at least onetemperature-compensating strain gauge is mounted on the block in alocation that does not deform in response to deformations of the sensortube.
 12. The probe according to claim 9, wherein the at least onetemperature-compensating strain gauge has a temperature-compensatingstrain gauge direction configured to be orthogonal to an axis of thesensor tube.
 13. A method for producing a medical probe, comprising:providing a flexible insertion tube, having a distal end for insertioninto a body cavity of a patient and which is configured to be broughtinto contact with tissue in the body cavity; installing a sensor tubecomprising an elastic material inside the distal end of the insertiontube, the sensor tube being configured to deform in response to forcesexerted by the tissue on the distal end; and fixedly attaching aplurality of strain gauges to a surface of the sensor tube at different,respective locations, the strain gauges being configured to generaterespective signals in response to deformations of the sensor tube. 14.The method according to claim 13, wherein the sensor tube is fixedlyattached inside the distal end to the insertion tube.
 15. The methodaccording to claim 13, wherein the surface of the sensor tube comprisesan outer curved surface of the sensor tube.
 16. The method according toclaim 13, wherein the respective signals are configured to generate amagnitude and a direction of a pressure on a termination of the distalend.
 17. The method according to claim 13, wherein the respectivesignals are configured to generate a magnitude and a direction of adeflection of a termination of the distal end.
 18. The method accordingto claim 13, wherein the plurality of strain gauges are symmetricallydisposed with respect to an axis of the sensor tube.
 19. The methodaccording to claim 13, wherein the plurality of strain gauges arepositioned on a circumference of the surface of the sensor tube, andwherein the circumference is centrally located with respect to thesensor tube.
 20. The method according to claim 13, wherein the pluralityof strain gauges have respective gauge directions, and wherein thegauges are fixedly attached to the surface of the sensor tube so thatthe respective gauge directions are parallel to an axis of the sensortube.
 21. The method according to claim 13, and comprising providing atleast one temperature-compensating strain gauge configured to generatesignals to compensate for changes in temperature of the plurality ofstrain gauges.
 22. The method according to claim 21, wherein the atleast one temperature-compensating strain gauge is mounted on a blockhaving a block-thermal mass, and wherein the sensor tube has anassembly-tube-thermal mass equal to the block-thermal mass.
 23. Themethod according to claim 22, wherein the at least onetemperature-compensating strain gauge is mounted on the block in alocation that does not deform in response to deformations of the sensortube.
 24. The method according to claim 21, wherein the at least onetemperature-compensating strain gauge has a temperature-compensatingstrain gauge direction configured to be orthogonal to an axis of thesensor tube.
 25. Apparatus for performing a medical procedure,comprising: a medical probe, comprising: a flexible insertion tube,having a distal end for insertion into a body cavity of a patient andwhich is configured to be brought into contact with tissue in the bodycavity; a sensor tube comprising an elastic material, contained insidethe distal end of the insertion tube and configured to deform inresponse to forces exerted by the tissue on the distal end; and aplurality of strain gauges fixedly attached to a surface of the sensortube at different, respective locations and configured to generaterespective signals in response to deformations of the sensor tube; and aconsole, which receives the respective signals from the strain gaugesand in response provides an indication of the forces on the distal end.26. A method for performing a medical procedure, comprising: providing aflexible insertion tube, having a distal end for insertion into a bodycavity of a patient and which is configured to be brought into contactwith tissue in the body cavity; installing a sensor tube comprising anelastic material inside the distal end of the insertion tube, the sensortube being configured to deform in response to forces exerted by thetissue on the distal end; fixedly attaching a plurality of strain gaugesto a surface of the sensor tube at different, respective locations, thestrain gauges being configured to generate respective signals inresponse to deformations of the sensor tube; receiving the respectivesignals at a console; and providing at the console an indication of theforces on the distal end in response to receiving the respectivesignals.